Nanowire sized opto-electronic structure and method for modifying selected portions of same

ABSTRACT

A LED structure includes a support and a plurality of nanowires located on the support, where each nanowire includes a tip and a sidewall. A method of making the LED structure includes reducing or eliminating the conductivity of the tips of the nanowires compared to the conductivity of the sidewalls during or after creation of the nanowires.

RELATED APPLICATION

This application claims the benefit of priority to U.S. ProvisionalApplication Ser. No. 61/719,133, filed on Oct. 26, 2012, the entirecontents of which are incorporated herein by reference.

FIELD

The present invention is directed generally to nanostructures and methodof making thereof and specifically to nanowire LEDs and methods ofmaking thereof.

BACKGROUND OF THE INVENTION

Nanostructures are promising building blocks for electronic andoptoelectronic semiconductor devices. The three-dimensional shape of thenanostructures can have challenges in device design. Differentcrystallographic planes can give different growth rate, materialcomposition, and doping.

SUMMARY OF THE INVENTION

In one aspect, the invention provides methods.

In certain embodiments, the invention provides a method for making a LEDstructure that comprises a support and a plurality of nanowires arrayedon the support, wherein each nanowire comprises a tip and a sidewall,wherein the method comprises reducing or eliminating the conductivity ofthe tips of the nanowires compared to the conductivity of the sidewallsduring or after creation of the nanowires. In certain of theseembodiments, conditions during or after creation of the nanowires arecontrolled such that the conductivity of the tips is reduced by at leastone order of magnitude compared to the conductivity of the tips withoutthe controlling of the conditions. In certain embodiments, thecontrolling of the conditions comprises forming an insulating layerafter the creation of the nanowires. In certain embodiments, theinsulating layer is formed over the array of nanowires so thatinsulation is layered on the tips and the sidewalls, then the array issubjected to etching under conditions that removes the insulating layeron the sidewalls but that leaves at least part of the insulating layeron the tips. In certain embodiments, the insulating layer comprisesSiO_(x), such as SiO₂. In certain embodiments, the condition thatremoves the insulating layer on sidewalls but leaves at least part onthe tips comprises etching, such as anisotropic etching. In certainembodiments, the nanowires comprise a first conductivity type core and asecond conductivity type shell, and the sidewalls and tips comprise thesecond conductivity type shell. In some of these embodiments, the shellis formed in successive layers such as successive layers of lowerconductivity material and higher conductivity material, wherein thehigher conductivity material preferentially forms on the sidewallscompared to the tips. In certain of these embodiments, the lowerconductivity material comprises p-AlGaN and the higher conductivitymaterial comprises p-GaN. In certain embodiments the shell is formed toprovide a plurality of the lower conductivity layers and a plurality ofthe higher conductivity layers on the sidewalls. In some of theseembodiments, the shell is formed to provide only a single lowerconductivity layer on the tips. In other embodiments, the shell isformed to provide a plurality of the lower conductivity layers and aplurality of the higher conductivity layers on the tips, wherein thethickness of the higher conductivity layers on the tips is less thanthat on the sidewalls, such that the tips are less conductive than thesidewalls, such as nonconductive. In certain embodiments where a coreand shell are constructed, the method comprises forming a highlyresistive material on the tips but not on the sidewalls of thenanowires. In some of these embodiments, the highly resistive materialis formed on the tips after the shell is completed. In other of theseembodiments, the highly resistive material is formed on the tips beforethe shell is completed. In certain embodiments, the highly resistivematerial comprises pGaN that comprises a very high Mg/Ga ratio and a lowVIII ratio. In certain embodiments where a core and a shell areconstructed, at least one layer of the shell is formed in a carrier gasthat comprises H₂, such as a carrier gas that comprises at least 50 sccmH₂. In certain embodiments, where the nanowires include a core and ashell, the first conductivity type semiconductor nanowire core isenclosed by the second conductivity type semiconductor shell for forminga pn or pin junction that in operation provides an active region forlight generation. The first conductivity type may comprise n-type, andthe second conductivity type may comprise p-type. In certainembodiments, a support comprises a n-type buffer layer from which thenanowire core is grown during production of the array of nanowires. Incertain embodiments, the support further comprises a dielectric maskinglayer, such that cores protrude from the buffer layer through openingsin the masking layer, and the shells are located on the masking layer.In certain embodiments, the support further comprises a substrate layerbeneath the buffer layer. In certain embodiments, the substrate layercomprises Al₂O₃. In certain embodiments, the support layer furthercomprises a reflective layer, such as Ag.

In another aspect, the invention provides structures.

In certain embodiments, the invention provides a LED structurecomprising a support and a plurality of nanowires arrayed on thesupport, wherein each of the nanowires comprises a tip and a sidewall,and wherein (i) the nanowires comprise a first conductivity typesemiconductor nanowire core and an enclosing second conductivity typesemiconductor shell, wherein the core and the shell are configured toform a pn or pin junction that in operation provides an active regionfor light generation; and (ii) the sidewall comprises a plurality oflayers that alternate a lower conductivity layer with a higherconductivity layer, and the tip comprises a single lower conductivitylayer or a plurality of layers corresponding to the sidewall layers,wherein the tip layers are thinner than the sidewall layers. In certainembodiments, the sidewall lower conductivity layers comprise p-AlGaN. Incertain embodiments, the sidewall higher conductivity layer comprisep-GaN. In certain embodiments, the tip lower conductivity layercomprises p-AlGaN. In certain embodiments, the first conductivity typecore is in electrical contact with a buffer layer of the support. Incertain embodiments, the second conductivity type shell is insulatedfrom the buffer layer by a mask layer. In certain embodiments, the firstconductivity type comprises n-type, the second conductivity typecomprises p-type. In certain embodiments, the tip layer or plurality oftip layers is between 10 and 30 nm thick.

In certain embodiments, the invention provides a LED structurecomprising a support and a plurality of nanowires arrayed on thesupport, wherein each of the nanowires comprises a tip and a sidewall,and wherein each of the tips comprises an outer insulating layer thatdoes not extend entirely down the sidewall. In certain embodiments, theinsulating layer comprises SiO_(x). In certain embodiments, the SiO_(x)comprises SiO₂. In certain embodiments, the insulating layer comprisespGaN that comprises a very high Mg/Ga ratio and a low VIII ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B schematically illustrate an exemplary nanowire LED withcurrent leakage.

FIG. 2 schematically illustrates an exemplary nanowire LED with multipleemission wavelengths.

FIG. 3 schematically illustrates a side cross sectional view of a basisof a nanowire LED in accordance with embodiments of the invention.

FIG. 4 schematically illustrates a side cross sectional view of ananowire LED structure on a buffer layer in accordance with embodimentsof the invention.

FIGS. 5A-C illustrate one embodiment of the methods and structures ofthe invention.

FIG. 6 illustrates one method of reducing current leakage in the tips ofnanowires.

FIGS. 7A and B illustrate nanowire structures with an insulating layerselectively located on the tips of the nanowires.

FIGS. 8A and 8B illustrate growth of a highly resistive material at thetip of the nanowire before p-GaN deposition.

FIGS. 9A and 9B illustrate growth of a highly resistive material at thetip of the nanowire after p-GaN deposition.

FIGS. 10A and 10B show a highly resistive structure grown on the tip ofthe nanowire before p-GaN deposition schematically (A) and in XSEM image(B).

FIGS. 11A-C illustrate a nanowire structure with multiple layers laiddown successively so that the sidewalls are multilayer and the tip is asingle layer, and electron conductance through the tip and sidewall.

FIG. 12 illustrates a multilayer nanowire structure with coalescedconductive layer.

FIG. 13 illustrates a multilayer nanowire structure with coalesced p-GaNstructure.

FIG. 14 illustrates electron micrographs of nanowire structures createdwith 0, 100, and 500 sccm H₂ in carrier gas.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides methods for altering the properties of selectedareas of nanowire based structures, in particular opto-electronicstructures such as LEDs, for example, nanowire light emitting diodes(LEDs), e.g., altering the properties to decrease conductivity ofselected portions of nanowires in a nanowire LED. The invention alsoprovides compositions that can be fabricated, e.g., using the methods ofthe invention.

The 3-dimensional nature of LEDs made from nanowires emerging from aplanar surface can present challenges in device design. Differentcrystallographic planes can give different growth rates, materialcomposition, and doping. This can, for example, cause leakage paths andmultiple emission wavelengths not desirable for the device. An exampleis a nanowire LED as illustrated in FIG. 1. In this example, a nanowireLED 100 includes a n-GaN core 101 in electrical contact with an n-GaNbuffer layer 103, intermediate layers of InGaN 105, GaN 107, and p-AlGaN109, overlaid with an outer layer of p-GaN 111, with a vertical sidewall113 and a conical tip 115. There are two growth planes, one on thesidewall 113, 10Ī0 region (also referred to as the m-plane herein), andon the tip 115, 10Ī1 region (also referred to as the p-plane herein);the p-GaN 111 has a very low growth rate in the 10Ī1 region, FIG. 1A. Ifa contact 117 covers the full nanowire 100, there can be a leak (shortcircuit) through the thin p-GaN layer 111, FIG. 1B. In addition, asshown in FIG. 2, the unequal distribution of p-GaN can result inmultiple emission wavelengths from the LED, as illustrated by theelectroluminescence spectra showing two peaks 201, 203 with a shorterwavelength from the m-plane (10-10) and a longer wavelength from thep-plane (10-11).

The methods and compositions of the invention are directed to alteringthe properties of the 10Ī1 region in order to reduce or eliminatecurrent flow through that region; in embodiments of the invention thisis achieved by controlling the growth of one or more layers in the 10Ī1region and in other embodiments this is achieved by altering thestructure of a finished 10Ī1 region.

In certain embodiments of methods provided by the invention, an array ofnanowires is grown on a support, where the nanowires comprise a core ofa first conductivity type and a shell of a second conductivity type; thenanowires have a tip, e.g., a conical tip (e.g., corresponding to the10Ī1 region, as described above), and sidewalls (e.g., corresponding tothe 10Ī1, as described above), which may be part of the secondconductivity type shell, and where the growth of the shell is controlledso as to produce a tip that has low or substantially no conductivity, sothat current leaks through the tip are reduced or eliminated. In some ofthese embodiments, this is accomplished by growing the shell as multiplesuccessive layers, where a nonconductive or lower conductivity layer isalternated with a higher conductivity layer, under conditions where thehigher conductivity layer grows slowly or not at all on the tip, so thatthe final tip of a nanowire is nonconductive or low conductivity and thesidewalls are conductive. In some of these embodiments, a nonconductiveor lower conductivity layer is grown on the tip but not the sidewalls ofthe nanowire. In some of these embodiments, a gas, e.g., H₂, isincorporated into a carrier gas that is used during some parts of thegrowth of the shell, such that materials are selectively deposited onthe sidewall but not the tip, and/or one or more conductive layers(e.g., InGaN) are etched away, allowing the tip to remain nonconductiveor low conductivity.

In certain embodiments, a nanowire array that has been grown underconditions that produce a conductive tip in the nanowires is subjectedto deposition of an insulating material and optional further treatmentso that the insulating material is confined to the tips and thesidewalls remain partially or completely free of the insulatingmaterial.

In certain embodiments, some or all of the above methods may be used incombination with deposition of, e.g., insulating or conductive materialthat is selectively layered on the tips and only partially layered onthe sidewalls, through angled travel of the material, to produce desiredproperties in the LED structure. Angled deposition of materials onnanoelectronic arrays, e.g., nanowire LED arrays, is described morefully in U.S. Provisional Patent Application No. 61/718,884 entitled“Nanowire Sized Opto-Electronic Structure and Method for Modifyingselected Portions of Same,” filed on Oct. 26, 2012, and herebyincorporated by reference herein in its entirety. In certainembodiments, some or all of the above methods are combined with laserablation of selected areas and selected structures of the nanoelectronicarray, e.g., nanowire LED arrays, to produce areas for, e.g., electrodeplacement. Laser ablation is described more fully in U.S. ProvisionalPatent Application No. 61/719,108 entitled “Nanowire LED Structure andMethod for Manufacturing the Same,” filed on Oct. 26, 2012, and herebyincorporated by reference herein in its entirety.

In certain embodiments, the invention provides a nanowire LED structurethat comprises an array of a plurality of nanowires, each of which has atip and a sidewall, where the sidewall is multilayered film, comprisinglayers of higher conductivity (e.g., p-GaN) and layers of lowerconductivity (e.g., p-AlGaN), in such a way as to allow conduction ofelectrons in the multilayer film, and the tip is either a single,nonconductive or lower conductivity layer or multilayered film in such away as to be nonconductive or lower conductivity than the sidewall.

In certain embodiments, the invention provides a LED structurecomprising a support and a plurality of nanowires arrayed on thesupport, where each of the nanowires comprises a tip and a sidewall, andwherein each of the tips comprises an outer insulating layer that doesnot extend down the sidewall. In certain embodiments, the inventionprovides a LED structure comprising a support and a plurality ofnanowires arrayed on the support, where each of the nanowires comprisesa tip and a sidewall, and wherein each of the tips comprises an outerinsulating layer that does not extend down the sidewall more than 1, 2,3, 4, 5, 7, 10, 15, 20, or 25% of the length of the sidewall, such asextending down 1-25% of the sidewall.

In the art of nanotechnology, nanowires are usually interpreted asnanostructures having a lateral size (e.g., diameter for cylindricalnanowires or width for pyramidal or hexagonal nanowires) of nano-scaleor nanometer dimensions, whereas its longitudinal size is unconstrained.Such nanostructures are commonly also referred to as nanowhiskers,one-dimensional nano-elements, nanorods, nanotubes, etc. Generally,nanowires with a polygonal cross section are considered to have at leasttwo dimensions each of which are not greater than 300 nm. However, thenanowires can have a diameter or width of up to about 1 micron. The onedimensional nature of the nanowires provides unique physical, opticaland electronic properties. These properties can for example be used toform devices utilizing quantum mechanical effects (e.g., using quantumwires) or to form heterostructures of compositionally differentmaterials that usually cannot be combined due to large lattice mismatch.As the term nanowire implies, the one dimensional nature is oftenassociated with an elongated shape. In other words, “one dimensional”refers to a width or diameter less than 1 micron and a length greaterthan 1 micron. Since nanowires may have various cross-sectional shapes,the diameter is intended to refer to the effective diameter. Byeffective diameter, it is meant the average of the major and minor axisof the cross-section of the structure. Although in the figures the nanoelements are shown to be pillar like and based on nanowire cores, i.e.,more or less “one dimensional” cores, it should be noted that the corescan also have other geometries such as pyramids with various polygonalbases, such as square, hexagonal, octagonal, etc. Thus, as used herein,the core may comprise any suitable nano element having a width ordiameter of less than 1 micron and a length greater than 1 micron andmay comprise a single structure or a multi-component structure. Forexample, the core may comprise a semiconductor nanowire of oneconductivity type or it may comprise the semiconductor nanowire of oneconductivity type surrounded by one or more semiconductor shells of thesame conductivity type and the core having a pillar or pyramid shape.For simplicity, a single component nanowire pillar core will bedescribed below and illustrated in the figures.

All references to upper, top, lower, downwards, etc., are made asconsidering the substrate being at the bottom and the nanowiresextending upwards from the substrate. Vertical refers to a directionparallel to the longer extension of the nanowire, and horizontal to adirection parallel to the plane formed by the substrate. Thisnomenclature is introduced for the ease of understanding only, andshould not be considered as limiting to specific assembly orientationetc.

In the methods of the invention, angled direction of a material to ananowire LED array is used in one or more steps of the methods toselectively alter the properties of certain parts of certain nanowiresin the structure while leaving other parts unaltered, e.g., altering theproperties of the tips of the nanowires to render them less conductivewhile leaving the conductivity of part or all of the sidewalls of thenanowires unchanged or substantially unchanged. The material directed tothe nanowire array may be, e.g., an insulator, or, e.g., a material thatalters the characteristics of selected surfaces of the nanowire, asdescribed more fully herein. The alteration in the conductivity, e.g.,decrease in conductivity, of the tips but not the sidewalls or a greaterdecrease in conductivity of the tip than the sidewall provides for lessleakage in the tips and more optimal light production from the nanowireLED array. In certain embodiments, laser ablation of selected portionsof the nanowire display may also be used to produce desired results, asdescribed more fully herein.

Any suitable nanowire LED structure as known in the art may be used inthe methods of the invention.

Nanowire LEDs are typically based on one or more pn- or p-i-n-junctions.The difference between a pn junction and a p-i-n-junction is that thelatter has a wider active region. The wider active region allows for ahigher probability of recombination in the i-region. Each nanowirecomprises a first conductivity type (e.g., n-type) nanowire core and anenclosing second conductivity type (e.g., p-type) shell for forming a pnor pin junction that in operation provides an active region for lightgeneration. While the first conductivity type of the core is describedherein as an n-type semiconductor core and the second conductivity typeshell is described herein as a p-type semiconductor shell, it should beunderstood that their conductivity types may be reversed.

FIG. 3 schematically illustrates the basis for a nanowire LED structurethat is modified in accordance with some embodiments of the invention.In principle, one single nanowire is enough for forming a nanowire LED,but due to the small size, nanowires are preferably arranged in arrayscomprising hundreds, thousands, tens of thousands, or more, of nanowiresside by side to form the LED structure. For illustrative purposes theindividual nanowire LED devices will be described herein as being madeup from nanowires 1 having an n-type nanowire core 2 and a p-type shell3 at least partly enclosing the nanowire core 2 and an intermediateactive layer 4. However, for the purpose of embodiments of the inventionnanowire LEDs are not limited to this. For example the nanowire core 2,the active layer 4 and the p-type shell 3 may be made up from amultitude of layers or segments. By controlling growth conditions thefinal geometry of a LED can range from elongated, narrow “pillarstructures” to relatively wide based pyramid structures.

In alternative embodiments, only the core 2 may comprise a nanostructureor nanowire by having a width or diameter below 1 micron, while theshell 3 may have a width or diameter above one micron.

For nanowire fabrication, the III-V semiconductors are of particularinterest due to their properties facilitating high speed and low powerelectronics. The nanowires can comprise any semiconductor material, andsuitable materials for the nanowire include but are not limited to: GaAs(p), InAs, Ge, ZnO, InN, GaInN, GaN, AlGaInN, BN, InP, InAsP, GaInP,InGaP:Si, InGaP:Zn, GaInAs, AlInP, GaAIInP, GaAlInAsP, GaInSb, InSb, Si.Possible donor dopants for e.g. GaP are Si, Sn, Te, Se, S, etc, andacceptor dopants for the same material are Zn, Fe, Mg, Be, Cd, etc. Itshould be noted that the nanowire technology makes it possible to usenitrides such as GaN, InN and AlN, which facilitates fabrication of LEDsemitting light in wavelength regions not easily accessible byconventional technique. Other combinations of particular commercialinterest include, but are not limited to GaAs, GaInP, GaAIInP, GaPsystems. Typical doping levels range from 10¹⁸ to 10²⁰. A person skilledin the art is though familiar with these and other materials andrealizes that other materials and material combinations are possible.

Preferred materials for nanowire LEDs are III-V semiconductors such as aIII-nitride semiconductor (e.g., GaN, AlInGaN, AlGaN and InGaN, etc.) orother semiconductors (e.g., InP, GaAs).

In order to function as a LED, the n-side and p-side of each nanowire 1has to be contacted, and the present invention provides methods andcompositions related to contacting the n-side and the p-side of thenanowires in a LED structure.

Although the exemplary fabrication method described herein preferablyutilizes a nanowire core to grow semiconductor shell layers on the coresto form a core-shell nanowire, as described for example in U.S. Pat. No.7,829,443, to Seifert et al., incorporated herein by reference for theteaching of nanowire fabrication methods, it should be noted that theinvention is not so limited. For example, in alternative embodiments,only the core may constitute the nanostructure (e.g., nanowire) whilethe shell may optionally have dimensions which are larger than typicalnanowire shells. Furthermore, the device can be shaped to include manyfacets, and the area ratio between different types of facets may becontrolled. This is exemplified in figures by the “pyramid” facets andthe vertical sidewall facets. The LEDs can be fabricated so that theemission layer formed on templates with dominant pyramid facets orsidewall facets. The same is true for the contact layer, independent ofthe shape of the emission layer.

The use of sequential (e.g., shell) layers may result in the finalindividual device (e.g., a pn or pin device) having a shape anywherebetween a pyramid shape (i.e., narrower at the top or tip and wider atthe base) and pillar shaped (e.g., about the same width at the tip andbase) with circular or hexagonal or other polygonal cross sectionperpendicular to the long axis of the device. Thus, the individualdevices with the completed shells may have various sizes. For example,the sizes may vary, with base widths ranging from 100 nm to several(e.g., 5) μm, such as 100 nm to below 1 micron, and heights ranging froma few 100 nm to several (e.g., 10) μm.

FIG. 4 illustrates an exemplary structure that provides a support forthe nanowires. By growing the nanowires 1 on a growth substrate 5,optionally using a growth mask, or dielectric masking layer 6 (e.g., anitride layer, such as silicon nitride dielectric masking layer) todefine the position and determine the bottom interface area of thenanowires 1, the substrate 5 functions as a carrier for the nanowires 1that protrude from the substrate 5, at least during processing. Thebottom interface area of the nanowires comprises the area of the core 2inside each opening in the dielectric masking layer 6. The substrate 5may comprise different materials such as III-V or II-VI semiconductors,Si, Ge, Al₂O₃, SiC, Quartz, glass, etc., as discussed in Swedish patentapplication SE 1050700-2 (assigned to GLO AB), which is incorporated byreference herein in its entirety. Other suitable materials for thesubstrate include, but are not limited to: GaAs, GaP, GaP:Zn, GaAs,InAs, InP, GaN, GaSb, ZnO, InSb, SOI (silicon-on-insulator), CdS, ZnSe,CdTe. In one embodiment, the nanowires 1 are grown directly on thegrowth substrate 5.

In embodiments in which a dielectric masking (growth mask) layer isused, the growth mask 6 may be patterned by photolithography to defineopenings for the nanowire growth, as described for example in U.S. Pat.No. 7,829,443, incorporated herein by reference in its entirety. In thisimplementation, the nanowires are grouped in an n-pad area, a non-activearea, a LED area (i.e., the area which emits light) and a p-pad area.However, embodiments of the invention are not limited to this. Forexample the p-pad area may be arranged on top of the nanowires formingthe light emitting part of the nanowire LED structure, whereby the p-padarea and the LED area coincide, as described in PCT InternationalApplication Publication Number WO 2010/014032 A1 to Konsek, et al.,published Feb. 4, 2010 and incorporated herein by reference in itsentirety.

Preferably, the substrate 5 is also adapted to function as a currenttransport layer connecting to the n-side of each nanowire 1. This can beaccomplished by having a substrate 5 that comprises a buffer layer 7arranged on the surface of the substrate 5 facing the nanowires 1, asshown in FIG. 4, by way of example a III-nitride layer, such as a GaNand/or AlGaN buffer layer 7 on a Si substrate 5. The buffer layer 7 isusually matched to the desired nanowire material, and thus functions asa growth template in the fabrication process. For an n-type core 2, thebuffer layer 7 is preferably also doped n-type. The buffer layer 7 maycomprise a single layer (e.g., GaN), several sublayers (e.g., GaN andAlGaN) or a graded layer which is graded from high Al content AlGaN to alower Al content AlGaN or GaN. The nanowires can comprise anysemiconductor material, but for nanowire LEDs III-V semiconductors suchas a III-nitride semiconductor (e.g., GaN, AlInGaN, AlGaN and InGaN,etc.) or other semiconductors (e.g., InP, GaAs) are usually preferred.The growth of nanowires can be achieved by utilizing methods describedin the U.S. Pat. Nos. 7,396,696, 7,335,908, and 7,829,443, andWO201014032, WO2008048704 and WO 2007102781, all of which areincorporated by reference in their entirety herein.

It should be noted that the nanowire 1 may comprise several differentmaterials (e.g., GaN core, InGaN active layer or one or more quantumwells (e.g., multiple quantum well, MQW, active region) and InGaN shellhaving a different In to Ga ratio than the active layer). In general thesubstrate 5 and/or the buffer layer Tare referred to herein as a supportor a support layer for the nanowires. In certain embodiments, aconductive layer (e.g., a mirror or transparent contact) may be used asa support instead of or in addition to the substrate 5 and/or the bufferlayer 7. Thus, the term “support layer” or “support” may include any oneor more of these elements.

The buffer layer 7 provides a structure for contacting the n-side of thenanowires 1.

The above description of an exemplary embodiment of a LED structure willserve as a basis for the description of the methods and compositions ofthe invention; however, it will be appreciated that any suitablenanowire LED structure or other suitable nanowire structure may also beused in the methods and compositions, with any necessary modificationsas will be apparent to one of skill in the art, without departing fromthe invention.

In certain embodiments, the invention provides methods of growing ortreating a nanowire LED structure to selectively alter thecharacteristics of parts of the structure. In certain embodiments, themethods are directed at reducing or eliminating current passage throughthe tip of a nanowire.

In certain embodiments, the invention provides methods of altering theproperties of a selective portion of nanowires in a LED nanowire array.In some of these embodiments, the invention provides a method forconstructing a LED structure, where the structure includes a support anda plurality of nanowires arrayed on the support, and where each nanowirecomprise a tip and a sidewall, by a method that includes controllingconditions during or after creation of the nanowires to reduce oreliminate the conductivity of the tips of the nanowires compared to theconductivity of the sidewalls. In some embodiments, the conditions arecontrolled in such a way as to reduce the conductivity of the tips by atleast one, two, three, four, or five orders of magnitude compared to thetips constructed without the controlling of the conditions, e.g.,constructed as detailed above for a nanowire LED array.

In certain embodiments, the conditions are such that an insulating layeris laid down after the creation of the nanowires. This can be done sothat the insulating layer is laid down, e.g., by sputter deposition, tocoat the tips and part or all of the sidewalls of the nanowires, thentreated to remove, or substantially remove, the insulating layer on thesidewalls but leave at least part of the insulating layer on the tips,e.g., so that the tips remain low conductivity or no conductivity, whilethe sidewalls retain conductivity. In certain embodiments, less than 20,15, 10, 7, 5, 4, 3, 2, or 1% (e.g., 0-25%) of the length of thesidewalls retains insulation. The nanowire is subjected to etching underconditions that removes the insulating layer on the sidewalls but thatleaves at least part of the insulating layer on the tips. Any suitableinsulator may be used. For example, in certain embodiments, SiO_(x), forexample SiO₂, may be deposited as the insulating layer. In certainembodiments, the removal of insulating material is accomplished byetching, e.g., by anisotropic etching. The etching may be controlled toremove the desired amount of insulating material, e.g., by controllingthe time of etch.

In certain of the embodiments of a method for constructing a LEDstructure, where the structure includes a support and a plurality ofnanowires arrayed on the support, and where each nanowire comprise a tipand a sidewall, by a method that includes controlling conditions duringor after creation of the nanowires to reduce or eliminate theconductivity of the tips of the nanowires compared to the conductivityof the sidewalls, the nanowires comprise a first conductivity type coreand a second conductivity type shell, and the sidewalls and tipscomprise the second conductivity type shell. Materials and techniquesfor construction of the cores and shells may be, e.g., as detailedherein. In certain embodiments, the shell is laid down as successivelayers, for example, successive layers of lower conductivity materialand higher conductivity material, e.g., by well-known epitaxialtechniques, where the higher conductivity material preferentially layerson the sidewalls compared to the tips. In certain embodiments, the lowerconductivity material comprises p-AlGaN and the higher conductivitymaterial comprises p-GaN. The shell may be laid down to provide aplurality of the lower conductivity layers and a plurality of the higherconductivity layers on the sidewalls. In certain embodiments, the shellis laid down (i.e., grown) to provide only a single lower conductivitylayer on the tips. In other embodiments, the shell is laid down toprovide a plurality of the lower conductivity layers and a plurality ofthe higher conductivity layers on the tips, wherein the thickness of thehigher conductivity layers on the tips is less than that on thesidewalls, such that the tips are less conductive than the sidewalls.The thickness and succession of layers is controlled so that theconductivity of the tip is greatly reduced or eliminated.

In certain embodiments where a core and a shell are constructed, theshell may be constructed so that a highly resistive material is laiddown on the tips but not on the sidewalls of the nanowires, or notsubstantially on the sidewalls of the nanowires. In some of theseembodiments, the highly resistive material is laid down on the tipsafter the shell is completed. In some of these embodiments, the highlyresistive material is laid down on the tips before the shell iscompleted. The highly resistive material may comprise, e.g., pGaN thatcomprises a very high Mg/Ga ratio and a low VIII ratio. For someembodiments, the preferred VIII ratio is about 10 to about 50 during thegrowth phase and preferred amount of Mg is about 2% to about 5%, i.e.,Mg to Ga ratio in the gas phase. It is well understood that the VIIIratio for typical GaN is greater than 3000 and Mg concentration is lessthan 1%, i.e., Mg to Ga ratio in the gas phase.

In certain embodiments where a core and a shell are constructed, atleast one layer of the shell is laid down in a carrier gas thatcomprises H₂, such as a carrier gas that comprises at least 50, 100,150, 200, 250, 300, 400, or 500 sccm H₂.

In certain embodiments where a core and a shell are constructed, thefirst conductivity type semiconductor nanowire core is enclosed by thesecond conductivity type semiconductor shell for forming a pn or pinjunction that in operation provides an active region for lightgeneration. In some of these embodiments, the first conductivity typecomprises n-type, the second conductivity type comprises p-type.

In certain embodiments where a core and a shell are constructed, and thecore comprises an n-type conductivity semiconductor, the supportcomprises a n-type buffer layer from which the nanowire core is grownduring production of the array of nanowires. In some of theseembodiments, the support further comprises a dielectric masking layer,such that cores protrude from the buffer layer through openings in themasking layer, and the shells are located on the masking layer. In someembodiments, the support further comprises a substrate layer beneath thebuffer layer, such as a substrate layer comprising Al₂O₃. In some ofthese embodiments, the support layer further comprises a reflectivelayer, such as a reflective layer comprising Ag.

In one exemplary embodiment as shown in FIGS. 5-7, an insulating layeris created to selectively insulate the 10Ī1 plane (p-plane) whileleaving the 10Ī0 plane unaffected or substantially unaffected. Aninsulating layer is deposited over a nanowire structure; the insulatormay be SiO_(x) (silicon oxide, e.g., SiO₂) or other suitable insulatingmaterial, such as Al₂O₃, iZnO, SiN, HfO₂, or the like, as known in theart. FIG. 5A illustrates an exemplary nanowire (NW) structure 100 withthin pGaN 111 on the 10Ī1 plane (p-plane); if a contact layer 117 isdeposited on the nanowire, then leakage paths 503 can be located in the10Ī1 plane (p-plane, also referred to as the tip 115 in FIG. 5B). If aninsulating layer 501 is deposited on the 10Ī1 plane (p-plane) beforedeposition of the contact layer 117, the leakage routes are reduced oreliminated (FIG. 5C). One method of creating the insulating layer 501 isshown in FIG. 6. A dielectric insulating material 601, e.g., SiO_(x)such as SiO₂, is deposited over the nanowire structure by any suitablemethod, such as sputter deposit. Because of the high aspect ratio of thenanowires, more insulating material is deposited on the 10Ī1 plane(p-plane, corresponding to the tip 115) than on the 10Ī0 plane (m-plane,corresponding to the sidewall 113). The insulating layer can then betreated by any suitable method to selectively remove the insulatinglayer, e.g., any suitable etching method, such as anisotropic(non-directional) etch, and the conditions controlled, e.g., the time ofthe etch, to allow removal of the insulating layer such that the thinnerinsulating layer on the sidewalls 113 is removed and the pGaN layer ofthe 10Ī0 plane (m-plane) is re-exposed but the tip 115 is still coveredwith a remaining portion 603 of the thicker insulating layer 501 (FIG.6). For a given method and nanowire structure the time of etch thatproduces optimal results may be determined, e.g., either by calculationor by empirical observation, or both, as known in the art. The result isan insulating layer 501 of, e.g., SiO_(x) such as SiO₂, on the top partof the nanowire, i.e., on the 10Ī1 plane (p-plane) with no insulator orlower insulator thickness on the 10Ī0 plane (m-plane). FIG. 7Aschematically shows the final nanowire product and FIG. 7B is an SEMimage of an array of nanowires, where the pGaN on the p-plane is aboutone half the thickness compared to the m-plane, with insulating SiO_(x)such as SiO₂, on the tips but not the sidewalls of the nanowires.

In a second exemplary embodiment as shown in FIGS. 8-9, an insulatinglayer 801 is grown during the growth of the nanowire 100. In thisembodiment, a III-nitride material that has significantly higherresistance than pGaN, e.g., pGaN with a very high Mg/Ga ratio and a lowVIII ratio during growth (a preferred amount of Mg is about 2% to about5%, i.e., Mg to Ga ratio in the gas phase and preferred VIII ratio isabout 10 to about 50) is selectively grown on the tip 115 compared tothe sidewalls 113 during the growth of the nanowire 100. This can bedone either before (FIGS. 8A and B) or after (FIGS. 9A and B) pGaN 111deposition, e.g., by epitaxial deposition. In either case, the result isa high-resistance (e.g., insulating) III-nitride layer 801 selectivelylocated at the top of the nanowires and not extending down the sidewalls113, or extending only minimally down the sidewalls 113 (e.g., thethickness of this layer 801 is at least two times thicker on the tip 115than on the sidewalls 113). This layer 801 greatly reduces or eliminatescurrent leakage at the tip 115 of the nanowire, compared to nanowireswithout the layer 801. A structure 801 grown before pGaN deposition isshown schematically in FIG. 10A and in XSEM image in FIG. 10B.

In a third exemplary embodiment as shown in FIGS. 11-13, successivelayers are deposited during the growth of the nanowire, and thesuccessive layering changes the conductivity on the tip compared to ananowire grown without successive layering. Without being bound bytheory, it is thought that pGaN grows more slowly in the 10Ī1 region(the p-plane) compared to the 10Ī0 region (the m-plane), but pAlGaNgrows at the same rate on both planes. By growing a multilayerstructure, e.g., of 2 nm pAlGaN/10 nm pGaN on the 10Ī0 region (them-plane), the 10Ī1 region (the p-plane) will end up with a thick pAlGaNlayer with no or very thin pGaN layer in between. The barriers are grownsuch that the holes and electrons are easily tunneling through thepAlGaN on the 10Ī0 region (the m-plane) but the combined pAlGaN layersform a barrier to electron travel on the 10Ī1 region (the p-plane). Inaddition, if a contact is placed on this structure, there will be poorcontact on the 10Ī1 region (the p-plane) pAlGaN, which is known to makepoor contact, and good contact with the pGaN on the 10Ī0 region (them-plane). The multilayer structure 1101 is illustrated in FIG. 11A witha conductive layer 117 over the entire structure 100, and electronmovement from the conductive layer 117 to the pGaN layer 101 of thenanowire is illustrated FIG. 11B, which shows that at the tip 115, thereis poor contact to the pAlGaN 109 and the combined pAlGaN layer 109 isthick enough that electrons cannot tunnel through, thus leakage(current) is reduced or eliminated in the tip, compared to the layer onthe sidewall 113 (FIG. 11C), which shows that there is Ohmic contact tothe pGaN 111 and that the pAlGaN 109 layers separating the thicker pGaNlayers 111 are thin enough for holes and electron tunneling (i.e., goodconductance and current). The contact and conductivity properties aretheoretical and it will be appreciated that the invention is directed tosuccessive deposition that leads to greatly reduced conductance at thetip 115 compared to the sidewalls 113 of the nanowire, no matter whatthe mechanism. The conductive layer 1101 is further illustrated in FIGS.12 and 13, which illustrate coalesced growth of pGaN 111 to provide aneasier contact deposition. FIG. 12 shows a multilayer 1101 nanowirestructure before coalesced pGaN deposition and FIG. 13 shows themultilayer structure after coalesced pGaN deposition. Methods for suchdeposition are described in, e.g., U.S. application Ser. No. 13/245,405,filed Sep. 26, 2011 and PCT Patent Application PCT/US12/51081 entitled“Coalesced Nanowire Structures With Interstitial Voids and Method forManufacturing Same,” filed Sep. 26, 2011, which are both incorporatedherein by reference in their entirety. Briefly, a first conductivitytype semiconductor nanowire core is grown from portions of asemiconductor surface of a support exposed through openings in aninsulating mask layer on the support, forming semiconductor activeregion shells on the cores, growing a continuous second conductivitytype semiconductor layer extending over and around the cores and theshells, such that a plurality of interstitial voids are formed in thesecond conductivity type semiconductor layer extending between the coresduring the step of growing, and forming a first electrode layer thatcontacts the second conductivity type semiconductor layer and extendsinto the interstitial voids.

A fourth exemplary embodiment as shown in FIG. 14. In this embodiment, afraction of the carrier gas for GaN barrier growth in the MQW activeregion is H₂ (e.g., H₂ is used in addition to an inert MOCVD carriergas). It has been observed that using H₂ as a fraction of carrier gasfor GaN barrier growth in a MQW active region will drastically reducethe growth rate of the MQW active region on the 10Ī1 region (thep-plane) (indicated by 1401 in FIG. 14) but will remain the same on the10Ī0 region (the m-plane) (indicated by 1403 in FIG. 14). Using H₂ inthe GaN barrier it is possible to achieve an m-plane only device havingInGaN light-emitting layer (e.g., MQW active region) selectivelydeposited on the m-plane but not on the p-plane. FIG. 14 illustrates theeffects of various concentrations of H₂ in the carrier gas, with adrastically reduced p-plane thickness and no quantum wells 1401 observedwith both 100 standard cubic centimeters per minute (sccm) and 500sccmH₂ in the carrier gas. In addition, there is little or no InGaN(which is not grown with H₂ carrier gas) on the p-plane, which waspresumably etched by the H₂ carrier gas during GaN deposition.

Further processing, such as laser ablation, laying down of a contact,and the like may be performed, for example as described in U.S.Provisional Patent Application No. 61/719,108 entitled “Nanowire LEDStructure and Method for Manufacturing the Same, filed on Oct. 26, 2012,and hereby incorporated by reference herein in its entirety. Forexample, after nanowires are formed or treated to decrease conductivityof the tips, an electrode layer, e.g., a transparent conductive oxide(TCO), such as ITO, may be deposited over the structure by any suitablemethod, e.g., sputter deposition, to make electrical contact with thep-GaN sidewalls of the nanowires that have not been coated with theinsulating material and provide a p-electrode. Laser ablation may beperformed to expose the nGaN buffer layer in certain areas and an N-sidemetal contact laid down on the exposed buffer layer to provide ann-electrode. The insulating layer on top of the nanowires acts toprevent or greatly reduce current leakage through the tips of thenanowires so that current is directed to the exposed areas of thesidewalls. This is merely exemplary, and any suitable method of formingelectrical contact with the pGaN and nGaN layers may be used.

A nanowire LED is intended to either emit light from the top of thenanowire, e.g., through the p-electrode, or from the bottom of thenanowire, e.g., through the support layer (e.g., through the conductivelayer and/or buffer layer and/or substrate) and this has to be takeninto account when choosing the contact materials. As used herein, theterm light emission includes both visible light (e.g., blue or violetlight) as well as UV or IR radiation. For a top emitting nanowire LED,as described in the above example, the top contact material needs to betransparent, e.g., ITO. A reflective layer, such as silver or aluminum,as described below, may make up part of the support. In the case of abottom emitting nanowire LED, the top contact material can be areflecting layer like silver or aluminum, as described below. In generalthe construction of a bottom emitting nanostructure entails providingreflective structure, such as a mirror, at or near i.e. adjacent the topportions of each individual light emitting nanoelement so as to directthe emitted light backwards through the buffer layer of the device.Bottom-emitting electrodes are described further in U.S. PatentPublication No. 2011/0309382, filed on Jun. 17, 2011 and PCT ApplicationNo. PCT/US11/40932, filed Jun. 17, 2011, both of which are incorporatedherein by reference in their entirety.

Silver, among the metals, has the best reflection coefficient in thevisible region of the optical spectra, but is more prone to exhibitcorrosion damage in normal atmosphere if not capped inside a structure.Si₃N₄, SiO₂, Al₂O₃ or any other stable dielectric can be used as acapping layer. Aluminum has a reflective index in the visible regionsomewhat lower than silver, but exhibits very good corrosion resistancein dry atmospheric environments. In order to improve device reliabilityadditional dielectric capping as described above may still be desired.In the case of a transparent top contact layer, Indium Tin Oxide (ITO),as described, or other transparent compounds or highly dopedsemiconductors having high electrical conductivity and transmittance maybe used.

Although the present invention is described in terms of altering theproperties of selected portions of nanowire LEDs, it should beappreciated that other nanowire based semiconductor devices, such asfield-effect transistors, diodes and, in particular, devices involvinglight absorption or light generation, such as, photodetectors, solarcells, lasers, etc., can be contacted in the same way, and in particularthe angled alteration method can be implemented on any nanowirestructures.

The invention also provides LED structures.

In certain embodiments, the invention provides a LED structurecomprising a support and a plurality of nanowires arrayed on thesupport, where each of the nanowires comprises a tip and a sidewall, andwhere (i) the nanowires comprise a first conductivity type semiconductornanowire core and an enclosing second conductivity type semiconductorshell, wherein the core and the shell are configured to form a pn or pinjunction that in operation provides an active region for lightgeneration; and (ii) the sidewall comprises a plurality of layers thatalternate a lower conductivity layer with a higher conductivity layer,and the tip comprises a single lower conductivity layer or a pluralityof layers corresponding to the sidewall layers, wherein the tip layersare thinner than the sidewall layers. In certain embodiments, thesidewall lower conductivity layers comprise p-AlGaN. In certainembodiments, the sidewall higher conductivity layer comprise p-GaN. Insome of these embodiments, the tip lower conductivity layer comprisesp-AlGaN. The first conductivity type core may be in electrical contactwith a buffer layer of the support. The second conductivity type shellmay be insulated from the buffer layer by a mask layer. In certainembodiments, the first conductivity type comprises n-type, such as nGaN,and the second conductivity type comprises p-type, such as p-GaN. Incertain embodiments, the tip layer or plurality of tip layers is between5 and 50 nm, for example between 5 and 40, such as between 10 and 40,e.g., between 10 and 30 nm thick.

In certain embodiments, the invention provides a LED structurecomprising a support and a plurality of nanowires arrayed on thesupport, where each of the nanowires comprises a tip and a sidewall, andwherein each of the tips comprises an outer insulating layer that doesnot extend down the sidewall, or extends no more than 1, 2, 3, 4, 5, 7,10, 13, 15, 20, 30, or 40% down the sidewall's length, e.g., no morethan 5%. In certain embodiments, the insulating layer comprises SiO_(x),such as SiO₂. In certain embodiments, the insulating layer comprisespGaN that comprises a very high Mg/Ga ratio and a low VIII ratio(preferred VIII ratio is about 10 to about 50 and preferred amount of Mgis about 2% to about 5%).

Though the embodiments have been described in terms of altering theproperties of the selected parts of the LED nanowires to render themless conductive, it will be appreciated that similar techniques may beused to render certain parts of LED nanowires more conductive, e.g.,conductive materials may be deposited to selectively contact thesidewalls, but not the tips, of nanowires.

In particular, it should be emphasized that although the figuresillustrate embodiments having a pillar like geometry and are based onnano wire core, i.e. “one dimensional” cores, it should be understoodthat the cores can have other geometries such as pyramidal shapes bychanging growth conditions. Also, by changing growth conditions, thefinal nano element can have a pyramidal shape, or any shape between apillar like and a pyramid shape.

All publications and patents cited in this specification are hereinincorporated by reference as if each individual publication or patentwere specifically and individually indicated to be incorporated byreference and are incorporated herein by reference to disclose anddescribe the methods and/or materials in connection with which thepublications are cited. The citation of any publication is for itsdisclosure prior to the filing date and should not be construed as anadmission that the present invention is not entitled to antedate suchpublication by virtue of prior invention. Further, the dates ofpublication provided may be different from the actual publication dateswhich may need to be independently confirmed.

The invention claimed is:
 1. A method of making a LED structure thatcomprises a support and a plurality of multilayer nanowires located onthe support, wherein each nanowire comprises a tip and a sidewall,wherein the method comprises depositing a GaN layer of the nanowires byMOCVD using a carrier gas that comprises H₂ to reduce or eliminate aconductivity of the tips of the nanowires compared to the conductivityof the sidewalls during creation of the nanowires.
 2. The method ofclaim 1, wherein the nanowires comprise a multilayer structure and theGaN layer of the nanowires is formed using a carrier gas that comprisesthe H₂ and an inert MOCVD carrier gas.
 3. The method of claim 2, whereinthe H₂ in the carrier gas is provided at 500 sccm or less.
 4. The methodof claim 1, wherein the GaN layer of the nanowires comprises a GaNbarrier layer for an active region of the LED structure.
 5. The methodof claim 4, wherein the active region comprises an InGaN light emittinglayer.
 6. The method of claim 5, wherein the GaN barrier layer isdeposited by the MOCVD on the InGaN light emitting layer.
 7. The methodof claim 5, wherein the InGaN light emitting layer is selectivelydeposited on a m-plane of the nanowires but not on a p-plane of thenanowires.
 8. The method of claim 5, wherein the InGaN light emittinglayer is deposited over a GaN core of the nanowires by MOCVD using acarrier gas that does not contain H₂.
 9. The method of claim 5, whereinthe InGaN light emitting layer is etched on a p-plane of the nanowiresduring deposition of the GaN barrier layer by the MOCVD using thecarrier gas that comprises H₂.
 10. The method of claim 5, whereindeposition of the GaN barrier layer by the MOCVD using the carrier gasthat comprises H₂ reduces a growth rate of the active region on ap-plane of the nanowire without reducing the growth rate of the activeregion on a m-plane of the nanowire.
 11. The method of claim 1, whereina conductivity of the tip is reduced by at least one order of magnitudeby using the carrier gas that comprises H₂ compared to the conductivityof the tip deposited without using the carrier gas that comprises H₂.